The present invention relates to a method of making an iron base magnetic material alloy powder containing at least 50% by mass of iron and also relates to a method of making a magnetic material product out of the powder.
The present invention is applicable to various iron base magnetic material alloys that include not only hard magnetic material alloys, but also soft magnetic material alloys and nanocomposite magnets including hard and soft magnetic phases in combination. The magnetic material products made by the method of the present invention cover a wide range including not just permanent magnets like sintered or bonded magnets, but magnetic shield materials as well.
In the known processes, various types of milling machines such as jet mills, power mills and ball mills have been used widely to pulverize magnetic material alloys. However, it is impossible to obtain a powder of a particle size as large as about 100 xcexcm using a jet mill. Using a power mill or ball mill on the other hand, the resultant particle size distribution cannot be a single normal distribution. For these reasons, a pin disk mill has been used most often to prepare a powder of a mean particle size of 10 xcexcm through 100 xcexcm by pulverizing a magnetic material alloy. Various techniques of pulverizing an iron base magnetic material alloy with a pin disk mill are disclosed in Japanese Laid-Open Publication Nos. 3-14203, 3-46202 and 10-321427, for example.
A pin disk mill is a kind of impact crusher. Normally, a pin disk mill includes two disks that are disposed to face each other. On one side of each of these two disks, multiple milling pins (which will be herein referred to as xe2x80x9cpinsxe2x80x9d simply) are arranged so as not to collide against each other. At least one of the two disks rotates at a high velocity. A workpiece to be pulverized by the pin disk mill is loaded into the space between the two disks. The workpiece collides against the pins on the rotating and/or non-moving disks and is pulverized due to the impact generated. The disks and pins of a pin disk mill are made of austenitic stainless steel (JIS SUS304), for example.
However, we found that if an iron base magnetic material alloy, containing at least 50% by mass of iron, is pulverized with a pin disk mill, the pins will be soon worn out during the pulverization process so that the particle size distribution of the resultant powder changes with time. An intermetallic compound contained in a nanocomposite magnet and a boride phase such as Fe23B6, in particular, have high hardness. Accordingly, these materials considerably wear the pins and blades of a milling machine, and the particle size distribution changes noticeably with time. When a milling machine like this is applied to manufacturing of magnetic material products including magnets, such a change in particle size distribution deteriorates the magnetic properties of the final products. For example, if the mean particle size increases as a result of the size distribution changing, then the percentage of magnetic powder particles successfully filled in might decrease in an injection or compaction molding process. In the current state of the art, that unwanted size distribution changing is unavoidable unless pins or other parts of the machine are replaced frequently. In that case, however, the throughput will decrease considerably. Additionally, the manufacturing costs will also rise because additional personnel costs are required for the exchange.
It is therefore an object of this invention to provide a method of making an iron base magnetic material alloy powder while avoiding both the short-term wear of pins and other parts and particle size distribution changing even when the material alloy is pulverized with a pin mill.
Another object of the present invention is to provide magnetic material products like bonded magnets by utilizing the inventive method of making an iron base magnetic material alloy powder.
An inventive method of making an iron base magnetic material alloy powder includes the steps of: a) preparing an iron base magnetic material alloy containing at least 50% by mass of iron; and b) pulverizing the magnetic material alloy using a pin mill. A portion of the mill, which comes into contact with the magnetic material alloy, is made of a cemented carbide material at least partially.
In one embodiment of the present invention, the cemented carbide material is preferably tungsten carbide.
In another embodiment, an iron base magnetic material alloy powder of a mean particle size of 10 xcexcm through 100 xcexcm may be obtained by pulverizing the magnetic material alloy using a pin mill.
In this particular embodiment, the step a) preferably includes: forming a melt of a material alloy; and quenching the material alloy to form a solidified alloy.
Specifically, the magnetic material alloy is preferably an Fexe2x80x94Rxe2x80x94B alloy, where Fe is iron, B is boron and R is a rare earth element. Part of boron atoms in the above alloy may be replaced with carbon atoms. R is preferably selected from the group consisting of Pr, Nd, Dy and Tb.
More specifically, the magnetic material alloy is preferably a nanocrystalline magnetic material for a nanocomposite magnet, for example. Alternatively, the magnetic material alloy may also be a soft magnetic material or a magnetostrictive material.
In still another embodiment, the pin mill preferably includes a rotating disk and multiple pins arranged on the disk, and at least part of the pins are preferably made of the cemented carbide material.
According to an inventive method of making a magnetic material product, a magnetic material product is formed out of an iron base magnetic material alloy powder that has been prepared by any of the embodiments of the inventive method of making an iron base magnetic material alloy powder.
In one embodiment of the present invention, the magnetic material product may be a permanent magnet such as a bonded magnet.
An inventive method of preparing an iron base alloy powder for a permanent magnet includes the steps of: a) cooling a melt of an Fexe2x80x94Bxe2x80x94R alloy by a quenching process to obtain a solidified alloy with a thickness of 80 xcexcm through 300 xcexcm; b) heat-treating and crystallizing the solidified alloy to impart permanent magnet properties to the alloy; and c) pulverizing the alloy using a pin mill to obtain a powder of a mean particle size of 10 xcexcm through 100 xcexcm. A portion of the pin mill, which comes into contact with the alloy, is made of a cemented carbide material at least partially.
In one embodiment of the present invention, the method may further include the step of coarsely pulverizing the solidified alloy before the step b) is performed.
In another embodiment of the present invention, before the step b) is performed, the solidified alloy may have a structure in which an amorphous phase; at least one metastable phase selected from the group consisting of Fe23B6, Fe3B and R2Fe23B3 phases; and an R2Fe14B phase co-exist. Alternatively, the solidified alloy may also have an amorphous structure before the step b) is performed.
In still another embodiment, the alloy imparted with the permanent magnet properties may be expressed by a formula Fe100-x-yRxBy where R is at least one rare earth element. R is preferably selected from the group consisting of Pr, Nd, Dy and Tb. In this formula, x is equal to or greater than 2 atomic percent and equal to or less than 6 atomic percent, and y is equal to or greater than 15 atomic percent and equal to or less than 20 atomic percent. The alloy preferably includes, as constituent phases, Fe, an Fexe2x80x94B alloy, and a compound with an R2Fe14B crystal structure. A mean crystal grain size of the constituent phases is preferably 100 nm or less.
An inventive method of producing a bonded magnet includes the steps of: preparing an iron base alloy powder for a permanent magnet by any of the embodiments of the method of preparing an iron base alloy powder; and molding the iron base alloy powder.